The present invention relates to a method of transforming an .alpha.-acyl pyrrole (II) having at least one hydrogen atom in the .beta. position into a .beta.-acyl pyrrole (I) as illustrated by the following: ##STR1## wherein R.sub.1 is a member selected from the group consisting of hydrogen, loweralkyl, and aryl. The identity and location of substituents X, Y, and Z are immaterial to the functioning of the method, provided that the substituent on at least one .beta.-position is hydrogen.
More particularly, the present invention relates to a method for preparing from the corresponding .alpha.-acyl pyrroles, .beta.-acyl pyrroles having the following formulas: ##STR2## wherein: R.sub.1 is as previously defined;
R.sub.2 is a member selected from the group consisting of hydrogen, loweralkyl, aralkyl, and aralkyl substituted with from one to three members each selected from the group consisting of loweralkyl, loweralkoxy, and halo, said aralkyl being a member selected from the group consisting of benzyl and phenethyl; and PA1 R.sub.3 is a member selected from the group consisting of hydrogen and loweralkyl.
The above compounds of formula (I) are useful as intermediates for useful ultraviolet light absorbing .beta.-substituted pyrroles of formula: ##STR3## wherein W is NO.sub.2 when R.sub.1 = H and W is CN when R.sub.1 = loweralkyl or aryl. Certain of the compounds of formula (I) wherein R.sub.1 is aryl are described in U.S. Pat. No. 3,644,631 as anti-inflammatory, analgesic, and anti-pyretic agents.
The ultraviolet absorbing pyrroles of formula (III) wherein W is NO.sub.2 and R.sub.1 is H may be prepared by Knoevenagel condensation of the .beta.-pyrrole carboxaldehyde of formula (Ic) with nitromethane as is known in the art. Those wherein W is CN and R.sub.1 is loweralkyl or aryl may be prepared by Wadsworth-Emmons condensation of the .beta.-pyrrole ketone of formula (Id) with diethyl cyanomethylphosphonate anion as is known in the art. These two reactions are illustrated below: ##STR4##
Typical ultraviolet absorption data are shown in Table I below. Because the predominant chromophore is the .beta.-substituted pyrrole olefin, the nature of the other substituents (X,Y,Z) has little or no effect on the absorption.
TABLE I ______________________________________ Compound Abs. Max. .epsilon. ______________________________________ ##STR5## 365 nm 16,917 ##STR6## 365 nm 17,818 ##STR7## 300 nm 20,500 ______________________________________
The subject compounds (III) strongly absorb ultra-violet (U.V.) light, generally above 280 nm, and are useful as U.V.-screening materials, for example, in plastic products and sunburn preventive formulations. Because of their general solubility in organic materials, the compounds may be used as U.V.-absorbers in plastics and resins such as, for example, polystyrene, polyethylene, polypropylene, polyacrylics (e.g., methacrylate resins, polyacrylamides, polyacrylonitrile fibers, etc.), polyamide (e.g., nylon) fibers, and polyester fibers. The inclusion of about 0.01-5.0 percent of the absorber, based on the polymer weight, is usually sufficient to render protection against U.V. light, such as in plastic films, light filters, etc. The absorber may be incorporated into the mixture of monomers before polymerization to form the polymer or it may be incorporated into the polymer at other stages during its handling, as by milling into the polymer together with other compounding ingredients, or during the spinning of the polymer into fibers, etc.